Liver-directed cell/gene therapy is of enormous significance for many genetic, metabolic and acquired conditions that may cause injury and/or adverse effects in the liver or multiple other target organs. Based on work in our and other laboratories over the past 25 years, hepatocyte transplantation has been performed in some 100 people with various conditions. These studies indicated that insights from rodent models were appropriate and predicted clinical outcomes in people after cell/gene therapy. Moreover, initial experiences of cell therapy in people established the need for returning to studies in animals, particularly to define mechanisms in engraftment of transplanted cells that could advance liver repopulation to achieve superior therapeutic outcomes. In more recent studies, it was shown that transplanted cells engrafted in the liver through multi-step processes. For instance, after deposition in liver sinusoids, transplanted hepatocytes engrafted in liver parenchyma, whereas after injection into the hepatic artery, transplanted hepatocytes were promptly destroyed. Entry of transplanted cells in the liver parenchyma required disruption of sinusoidal endothelial barrier, so that cells could travel through the space of Disse, followed by liver remodeling to accept transplanted cells in the parenchymal structure. However, cell transplantation induced ischemia and tissue injury, leading to recruitment of inflammatory cells in the setting of syngeneic or autologous cells, and of professional immunocytes, in the setting of allogeneic cells, along with activation of liver sinusoidal endothelial cells and hepatic stellate cells. Cell-cell interactions in the liver could be beneficial, e.g., disruption of liver sinusoidal endothelial cells improved engraftment of transplanted cells and beneficial activation of hepatic stellate cells released cytoprotective factors and promoted matrix remodeling, again with superior cell engraftment. Or cell-cell interactions could be deleterious. For instance, release of inflammatory cytokines and chemokines resulted in clearance of significant fractions of transplanted cells over several hours, while activation of additional adaptive immune responses against allogeneic cells, led to the clearance of cells over several days. As suitable interventions could alter these initial cell losses, we will address the hypothesis that insights into mechanisms of cell transplantation-induced liver inflammation and its effects on cell engraftment will help obtain effective pharmacological approaches for advancing cell/gene therapy. To understand the nature of cellular perturbations, we will perform studies in superb rodent models and define inflammatory cell-specific changes, including mechanisms in relevant cytokine-chemokine activations, as well as cell-cell interactions. We will perform pharmacological studies with clinically-relevant drugs to block undesirable mechanisms for improving engraftment of transplanted cells. Moreover, we will examine the efficacy of these manipulations for liver repopulation and cell therapy in further animal models. Taken together, this proposal will advance novel insights for liver-directed cell/gene therapy and will be of extensive significance for human health.